People should aim to decrease their sitting time by 2-3 hours in a 12-hour day, according to the authors.
Although it is common knowledge that a sedentary lifestyle brings bad health, the extent to which this may be mediated by regular exercise was less well known.
"What we didn't know was whether the sitting time and health relationship was because people were also exercising poorly," senior author Dr. David Alter - of the Toronto Rehabilitation Institute-University Health Network (UHN) and Institute for Clinical Evaluative Sciences in Canada - told Reuters Health.
Dr. Alter and colleagues conducted a meta-analysis of 47 studies that tracked groups of people who reported data on how often they spent sitting and on how much exercise they took.
They found that people who were the most sedentary were more likely to be diagnosed with type 2 diabetes, cardiovascular disease and various cancers, including breast, colon and ovarian cancers. The most sedentary people were also 24% more likely to die during the studies than participants who spent the least amount of time sitting.
Although this association was more pronounced among people who spent little time exercising, the study also found that prolonged sitting time was associated with poor health outcomes, regardless of physical activity.
"Another way of saying it is just because one does their 30 to 60 minutes of exercise per day [it] doesn't ensure their health," Dr. Alter says in a UHN news release. "These are two distinct factors, we need both, we need exercise and need to be sitting less."
Standing, the study points out, burns twice as many calories as sitting. Therefore, spending less time sitting and more time standing could be a strategy - separate to exercise - that may be helpful at reducing disease risk.
The average person spends half of their day sitting
"More than one half of an average person's day is spent being sedentary - sitting, watching television, or working at a computer," Dr. Alter says. "Our study finds that despite the health-enhancing benefits of physical activity, this alone may not be enough to reduce the risk for disease."
Currently, public health guidelines suggest that adults should walk at least 30 minutes per day. But Dr. Alter says that "it is not good enough to exercise for 30 minutes a day and be sedentary for 23 and [a] half hours."
He says that people should aim to decrease their sitting time by 2-3 hours in a 12-hour day, suggesting that standing during the commercial breaks on TV, or working standing up at your desk for a couple of hours a day may be beneficial.
However, Dr. Alter emphasizes that these strategies do not replace daily exercise. Rather, because the "health hazards are accelerated quite markedly" among people who do not exercise, reducing sitting time is most important for this group.
Also, Dr. Alter reminds that none of the studies in the meta-analysis are randomized controlled trials. As such, the authors are only able to report an association between sitting time and increased risk for disease, rather than being able to conclusively state that sitting directly causes disease.